1. Field of the Invention
The present invention relates to a process for producing an inorganic biomaterial which is useful as an implant material for artificial bones, dental implants, etc.
2. Description of Prior Art
Ceramics have been attracted public attention because ceramics are considered to be biomaterials harmless to a living body compared with polymers and metallic materials. In recent years, ceramics as biomaterials make remarkable progress. Of ceramics, bioactive ceramics capable of forming a chemical bonding with bones are known. Such bioactive ceramics are united with a living body so that there arises no loosening problem. As the bioactive ceramics, there is known crystallized glass obtained by precipitating an apatite crystal [Ca.sub.10 (PO.sub.4).sub.6 (O.sub.0.5, F).sub.2 ] and a wollastonite crystal [CaSiO.sub.3 ]. However, the bending strength of the crystallized glass shows a value within a range of about 120 to about 230 MPa. To improve the bending strength of the crystallized glass, bioactive materials such as composite crystals of bioactive crystallized glass and zirconia ceramics and composite crystals of bioactive crystallized glass and alumina ceramics have been developed (Japanese Patent Unexamined Publication Nos. Sho-62-231668 and Sho-63-82670). Those composite materials show relatively high bending strength in a range of from 230 to 350 MPa. However, those values are not yet fully satisfactory from the standpoint that the above materials are used in applications such as artificial bones and dental implants. Accordingly, the above materials are subjected to considerable restriction in the purpose of use thereof.
As means for obtaining a material of higher strength, for example, Japanese Patent Unexamined Publication Nos. Hei-1-115360 and Hei-1-115361 disclose a process for producing an inorganic material comprising the steps of mixing glass powder having a particle size smaller than 75 .mu.m, with zirconia or alumina ceramic powder having a particle size smaller than that of the glass powder, molding the resulting mixture into a desired shape, sintering the glass portion of the resulting compact to crystallize it, and sintering the zirconia or alumina ceramic powder portion.
The processes disclosed in the Japanese Patent Unexamined Publication Nos. Hei-1-115360 and Hei-1-115361 are useful in the case where the content of zirconia or alumina ceramic powder is large. However, the processes have the following disadvantages in the case where the content of zirconia or alumina ceramic powder is small. That is, when the glass powder is heated to a temperature enough to sinter the glass powder, (1) the zirconia or alumina ceramic powder may be enveloped in the glass being fluidized, and/or (2) in the positions where the glass is not fluidized well, pores may be formed easily in the vicinity of the interface between the glass and the zirconia or alumina ceramic powder. When the glass is further heated to a temperature enough to crystallize the glass, the fluidization of the glass stops to start crystallization while the states of (1) and (2) are kept as they are with no change. Accordingly, when the zirconia or alumina ceramic powder is heated to a temperature enough to sinter the ceramic powder, the zirconia or alumina ceramic powder cannot be sintered in the positions where the state of (1) exists so that a skeleton of zirconia or alumina ceramics having high strength cannot be formed. Consequently, the resulting composite material cannot show high strength. On the other hand, pores remain in the composite material in the positions where the state of (2) exists, so that the resulting composite material cannot show sufficiently large strength.
Accordingly, in the processes disclosed in the Japanese Patent Unexamined Publication Nos. Hei-1-115360 and Hei-1-115361, the content of crystallized glass contributing to bioactivity must be set to be smaller than the content of zirconia or alumina ceramics as a reinforcement material. In short, bioactivity must be sacrificed. Hence, the processes have a disadvantage in that a considerable time is required for forming a chemical bonding with bones.
As another means for improving the strength of ceramic materials, Japanese Patent Unexamined Publication No. Sho-64-18973 discloses a process in which a ceramic composite material of high strength is prepared by adding partially stabilized zirconia powder with a particle size of 1 .mu.m or less and metal fluoride powder to calcium phosphate powder, such as apatite powder, .beta.-tricalcium phosphate powder and the like, with a particle size of 1 .mu.m or less.
FIG. 5 typically shows the result of observation, through an electron microscope, of the ceramic composite material disclosed in the Japanese Patent Unexamined Publication No. Sho-64-18973. In the drawing, metal fluoride is not shown because the content of metal fluoride is very small. As is clear from FIG. 5, the ceramic composite material has a structure in which fine crystals of calcium phosphate 13 with a particle size of 1 .mu.m or less and fine crystals of partially stabilized zirconia 14 with a particle size of 1 .mu.m or less were mixed together at random. In general, zirconia and calcium phosphate are apt to react with each other. When the two components are mixed together at random as described above, the surface area where the two components are in contact with each other becomes so large that the two components react with each other easily. When calcium phosphate and partially stabilized zirconia react with each other as described above, calcium phosphate and partially stabilized zirconia form solid solutions. As this result, the amount of calcium phosphate is reduced so that excellent biocompatibility cannot be obtained. Further, partially stabilized zirconia is fully stabilized by reaction with calcium phosphate so that the strength and toughness of the ceramic composite material obtained are often unsatisfactory.